1. Field of the Invention
The invention relates to a process for the production of sintered material, especially abrasive grain, based on aluminum oxide made from aluminum hydroxide (gibbsite or hydrargillite) or aluminum oxide calcined at a lower temperature.
2. Background Art
.alpha.-Aluminum oxide (corundum), because of its great hardness, has been used for many decades as an abrasive. The standard process for the production of corundum suitable for grinding purposes composes in melting aluminum oxide (alumina) or raw materials containing aluminum oxide (bauxite) in an electric arc furnace and crushing and screening the solidified molten mass to the desired abrasive grain size. Both melting and size reduction use a lot of energy because of the high melting point and great hardness, and require units that are expensive to acquire and maintain. Moreover, the properties of the abrasive grains thus obtained, especially their toughness, are not optimal for many uses.
Therefore, efforts were already undertaken early to obtain corundum abrasive grains by heating compounds containing aluminum below the melting point of corundum (approximately 2050.degree. C.). However, in this connection, it turned out that it was not only important that the material to be sintered be dense and nonporous, but that a decisive role be played above all by the microstructure of the sintered material. It is especially important that a uniformly fine texture with crystallite sizes of .ltoreq.1 .mu.m result without including some coarse crystals.
The known solutions achieve this aim by the so-called sol-gel process by using highly pure boehmite (aluminum oxide-monohydrate, AlOOH) as the initial material (European Published Patent Application No. 24099) and optionally adding crystallization seeds which prevent the formation of large crystallites, by making possible a quick, complete crystallization in the desired modification, so that at a lower temperature there is neither time nor space for excessive growth of the individual crystallites (European Published Patent Application No. 152768). The thus-obtained products actually are qualitatively of very high grade, but relatively expensive, since the initial material is produced by hydrolysis of aluminum alkoxides which, on their part, are quite expensive.
The necessary low content of alkali metals, especially sodium, however, can hardly be achieved in any other way. A low sodium content is especially important to avoid the formation of .beta.-aluminum oxide during heating. The latter has an especially disadvantageous effect on the abrasive properties because it is formed as coarse crystals. It was also attempted to obtain abrasives of comparable quality starting from less high grade boehmite, which, however, was attained only by the addition of considerable amounts (atom ratio to aluminum 1:35 to 1:2) of sintering auxiliary agents and adhering to a specific rate of heating (West German Patent No. 3,219,607). However, by these additions, additional solid phases are formed, for example, the spinels already described in above-mentioned European Published Patent Application No. 24099, which are undesirable because they make the abrasive grain "softer".